Mnemonic for remembering that चाकू cākū means knife and चम्मच cammac means spoon: चाकू sounds sharper. For comparison, Spanish cuchillo 'knife' sounds sharper than cuchara 'spoon' or tenedor 'fork'. (Ideophone, is the technical term.)
Because there's no ek chaakoo so it cant translate to a knife. In Hindi, the objects in those sentences are always particular as illustrated in Engish with the help of the unless they don't have a ek in the sentences.